1. Field of the Invention
The present invention relates to a process for the hydroxylation of phenols or phenol ethers, and, more especially, to the hydroxylation of phenols/phenol ethers by reaction with hydrogen peroxide in the presence of a catalytically effective amount of titanium dioxide.
2. Description of the Prior Art
The hydroxylation of phenol or substituted phenols utilizing hydrogen peroxide, to prepare diphenols, is a known reaction.
French Patent No. 69/45,467, published under No. 2,071,464, describes a process in which the reaction is catalyzed by a strong acid such as, for example, perchloric acid or sulfuric acid.
German Patent No. 2,410,742 describes a process similar to the above, wherein the hydrogen peroxide is employed in the form of a virtually anhydrous organic solution.
These two processes are of considerable interest, and the first is carried out industrially.
In recent years, however, attempts have been made to catalyze the hydroxylation reaction using solids that are not dissolved in the reaction medium, in order to simplify their separation from said medium of reaction and to permit their possible recycling, as well as to avoid the by-product salts which are in most cases formed during the removal of dissolved acid catalysts.
Thus, French Patent Application No. 81/17,023 (published under No. 2,489,816) describes the use of titanium silicalite as a heterogeneous catalyst in the hydroxylation of aromatic compounds by reaction of hydrogen peroxide therewith.
The small size of the particles of the catalyst used renders their separation from the reaction medium very difficult and causes problems in the recycling thereof, even though it is essential in an industrial process to recycle a costly catalyst.
In order to overcome this problem of catalyst separation, it has been proposed, in European Patent Application published under No. 200,260, to use agglomerates of these fine particles of titanium silicalite.
It has also been proposed, in European Patent Application EP-A-0,299,893, to use bridged clays as such hydroxylation catalysts. Even though interesting results are obtained by conducting the process in such manner, research on the heterogeneous catalysis of the hydroxylation of phenols or phenol ethers using hydrogen peroxide is continuing.